The Beginnings of Fatigue

Considering my last post, I thought I would blog a bit more about the start of the history of fatigue. As an engineer I find the study of how to safely design and construct things very interesting; but I find the history of science and engineering that led to that understanding perhaps more interesting still. As is the case in all areas of scientific study (chemistry, biology, physics, etc.) the study of engineering follows the story of the successes and failures of those attempting to understand the rules and principles of the natural world. Fatigue is just one of a near infinite supply of examples.

Fatigue was first conceptualized in 1829 by a German mining administrator named Wilhelm Albert who noticed that the iron chains used to hoist carts from the mine would break from small, repeated loads even if they were not accidentally overloaded. Although Wilhelm's training was in law, he built a machine that would repeatedly cycle a load on a chain and found that the failure of the chain was due to the number of loads the chain experienced rather than the load itself. He published a paper on this concept in 1837 and the idea of mechanical fatigue was born.

While the idea of fatigue was now out there, Wilhelm's paper changed very little about the way humanity operated on a day to day basis. Some research was done but as is the general rule regarding our species, we did not see the need to understand something until after a horrific tragedy resulting from neglecting it. On Sunday May 8, 1842 there was a catastrophic failure of the front axle of the primary steam locomotive pulling 16-18 carriages in an event that became known as the Versailles Rail Accident. This caused the locomotive to derail and set off a chain of events that led to the fire box flipping over and a total pile-up of the passenger carriages. Needless to say, the practice of locking passengers in their carriages while the train was in motion did little to help matters as somewhere between 50 and 200 people (sometimes whole families traveling together) died in the aftermath.

Artist Depiction of the Accident (Source - http://en.wikipedia.org/wiki/Versailles_rail_accident)

This sparked an uproar in France. Newspapers were sent letters with criticisms and suggestions. Some religious groups suggested that God was punishing the wicked for traveling on the Sabbath (some things never change). Meanwhile, French engineers got to work attempting to understand the unique failure that led to the tragedy. The French government started a commission to investigate the cause of the failure; this commission recommended a procedure of monitoring and testing axles to determine their service life. The basis of these practices are also applied today to the frames of airplanes (who also undergo cyclic loading via the pressurized cabin of the plane). Legendary scientist and thermodynamics co-founder William Rankine (who I just learned was actually Scottish, not French) helped the movement to find a solution for safe management of railway axles. Although many scientists worked to research the idea of fatigue, German engineer August Wöhler and Scottish engineer William Fairbairn launched perhaps the most comprehensive investigation of fatigue between 1860-1870. Though I could spend a whole blog posting on their findings alone, the basic summary is that they found that the range of cyclic loading is more important that the peak stress that occurs from the load. They also introduced the extremely important idea of an Endurance Limit: the idea that for some materials, there is an amount of stress under which failure will "never" occur regardless of the amount of cycles of loading. Above that? Haha, well things get complicated but more on that in the next post.